Comparative statics of joint reproductive allocation

We perform a perturbation analysis (comparative statics) of how optimal rep roductive effort and per offspring investment are jointly affected by diffe rent selective factors. The factors considered are: (1) mortality sources, classified according to affected stage (juvenile or adult) and to its natur e (avoidable or unavoidable), and (2) resource (energy) availability for th e adult individual. The joint approach reveals both direct and indirect eff ects of each selective pressure. These interactive effects spring from the nonlinearity of reproductive expenditure, separated into a part devoted to endowing offspring (provisioning cost) and another part invested to make re production possible (requisite cost). The latter is envisioned as a reverse sigmoid function of fecundity (most models, so far, have considered only t he first kind of cost). The indirect effects have the consequence of enlarg ing the class of selective pressures that can induce changes of offspring s ize and clutch size, as compared with current explanations. So, they illumi nate new causes for some effects, and show new effects for some well-known selective causes. Several joint patterns in the two variables, shown by ani mals and plants in the field, can thus be given more appropriate interpreta tions than traditional, piecewise, ones. (C) 2000 Academic Press.